Katsuo Okumura

Distribution of environmental cesium-137 in tree rings

Abstract Environmental 137 Cs concentrations in tree rings grown since 1914 and 1929 were measured and compared with the record of 137 Cs deposition from weapons fallout. 137 Cs was found in tree rings formed prior to 1945 when the first nuclear weapon test was conducted. The radiocesium record in wood is not correlated with that of fallout deposition.

Characteristics of negative corona discharge using CWT with and without airflow

The Trichel pulse (TP) corona generated in the negative corona discharge has a regular interval and repetition of similar waveforms in nonuniform electric field in a needle-plane gap. An electron avalanche causes the TP. Its repetition is determined by accumulation of negative ions near a needle tip. The electric field intensity near a needle tip is weakened by the negative ions accumulated near the tip. Hence, it is required to clarify the influence of accumulated negative ions on the electric field intensity. We propose to use the continuous wavelet transformation (CWT) technique to study the dependence on both voltage and airflow, and the characteristics of the TP. In this study, the waveform of the TP is measured with a velocity of airflow from 0 to 7 m/s. From the experimental results, it is shown that the region of accumulated negative ions exists within 1 mm from the tip of needle electrode. Therefore, to understand the effect of accumulated negative ions near a needle tip, the authors investigated the influence of accumulated negative ions with and without airflow blown within 1 mm from the tip of the needle electrode using CWT. The repetition rate decreases with decreasing the applied voltage, and with increasing the velocity of airflow in front of the needle tip. The wavelet coefficients with airflow are larger than without airflow at 100 and 250 MHz; but 10 MHz, and the pulse interval for the wavelet spectrum between peaks with airflow is wider than without airflow. The characteristics of the TP waveform are well explained with both the wavelet coefficient and the averaged wavelet spectrum peak interval

A typical noiseless Pockels E field sensor

A new practical Pockels E field sensor has been demonstrated. By combining a ferroelectric liquid crystal and a polarization-maintaining optical fiber with the sensor, it is theoretically found that almost all of the optical and electronic noise produced in the system can be completely eliminated. This fact has been experimentally proven not only under the conditions of a uniform electric field but also those of a nonuniform electric field and with ac voltages applied to a stabilized corona discharge. It is shown that if with the new Pockels E field sensor, the smallest detectable phase change was set to be 10(-5) rad, the minimum measurable E field in the electrical discharge could be reduced from 10(2) to 10(-1) kV/m. This technique should be useful not only for the field of electrical discharge studies but also for other optical measurement applications such as length, temperature, and pressure using appropriately configured transducers.